The so-called "Ediacaran period" is a fascinating but mysterious period of animal evolution. It is of importance to researchers because it's the main period just before the Cambrian. The Cambrian brought about just about all the basic animal forms that we are familiar with today: arthropods, which includes crabs, trilobites, and later insects; proto-fish which led to fish, amphibians, reptiles, and mammals; segmented worms such as earth-worms; cephalopods, which led to octopuses, and other phyla that still exist today.

But the critters of the Ediacaran period generally seem to not match the Cambrian critters very well. They are mostly soft-bodied, lack legs and limbs, and strangely most don't seem to have a digestive track. It's as if all the Ediacaran were wiped out by some big catastrophe to be replaced by the Cambrian forms, the familiar body plans. We know a fair amount about how Cambrian forms ate and lived because we have their modern counterparts to study. But all we have from the Ediacaran period is just fossil imprints and hints of tracks created in the mud by movement.

A typical Cambrian animal is based on the "hollow tube model" where one end of the tube is the mouth and the other end the anus. Food goes into the mouth, gets digested inside the "tube", and the waste products pass out through the anus. We take this form for granted because the majority of all existing animals use some variation of this formula, including the human body. However, perhaps being overly familiar with this tube plan has blinded us to other possibilities. Alternatives may be a clue to the strangeness of the Ediacaran critters. (I call them "critters" because they may not even be animals. "Critters" is informal enough to avoid prematurely classifying them.)

Some Ediacaran forms are odd blobs with flat bottoms that in some ways resemble shell-less gastropods (the snail and slug family). But, a good many of the Ediacaran critters are nearly entirely covered with or are built with fairly-small ridges, resembling a magnified bird feather, a hair comb, or the bottom side of a mushroom head.

Critters that have these ridges include both (probably) free-crawling and stationary forms that attach themselves to rock or the sea floor. These ridges apparently were an important innovation to the Ediacarans because many varied forms possess them. The ridges are often interpreted to imply they were filter feeders because ridges increase the surface area of the critter, allowing it to absorb or filter more food.

But there is another possibility besides filter feeding. The ridges may be an inverted digestive system. Their digestive system could be on the outside. This is perhaps why they don't seem to fit the tube digestion model of Cambrian forms; the tube model may be a later innovation.

A typical scenario could be as follows. A ridged Ediacaran would crawl up on top of a stationary or slower life-form. It would then release digestive juices which would soften and liquefy the body of its victim. It then sucks in the liquefied body remnants when they are soft enough. The ridges provide more surface area to absorb the victim.

Existing life-forms perform a variation of this by storing at least two compounds that together form digestive enzymes. But these compounds are kept apart in the organism itself until it is ready to use them. They are only squirted together on the digestive target or enemy. Otherwise, they would digest the owner also.

External digestion is uncommon in modern life-forms, but it does exist. Some echinoderms extend their stomach outside their body to digest their victim. Some insects also inject digestive enzymes into the victim's body to aide or hasten the internal digestive process, which reduced the chomping needed to break it down.

The advantage of external digestion is that you can "eat" fairly large creatures. You don't have to bite, chew, or swallow. You just hop on top of your dinner and start digesting. If it is docile, your dinner may even be far larger than you because you don't have to digest it all at once. The down-side is that you cannot "bite and run". The later tube model we talked about allows one to grab a big bite and then run off to hide while still digesting internally.

The downside of the tube model is that your meal has to fit inside you. You can only do this by expanding your body, chewing the food into smaller pieces, or eating only things small enough to swallow. Expanding your body makes you slow and cumbersome; biting and chewing requires a fair amount of energy; and eating smaller food limits your diet.

From our perspective, though, the tube model appears to be the obviously superior design because one can escape predators more easily. However, familiarity may have blinded us to the external-digested possibilities. The tube model may not be such a clear slam-dunk. The ridged Ediacarans remained a dominate form for tens of millions of years without being noticeably bothered by or replaced by the tube model. It is often suggested that this is because later forms of life (the tube-based Cambrians) just had not evolved yet. But it may be that they couldn't yet compete on a large scale.

The Ediacarans may have been chemical warriors that rivaled the power of any challengers. If something comes to chew on you, you just release powerful digestive juices to scramble your enemy's body or mouth. These juices may have been supplemented with poisons over time. Poison has been a very effective defensive and offensive tool in existing animals even. Poisonous snakes, insects, scorpions, and jellyfish haunt us today. In many ways they are more dangerous to us than large predators such as sharks and bears. A certain species of centimeter-sized box jellyfish have killed swimmers. Why pick a physical fight when a chemical one is easier? The scorpion's poison may be the reason it has lasted nearly unchanged for 300-million years.

Thus, the Ediacaran form of chemical offense and defense may have nullified any selective advantage of the tube model. They may not be the primitive blobs that they are often characterized as. Chemical weapons do not fossilize nearly as easily has physical structures, so they may have had complex chemical arsenals that we simply cannot see in fossils. The frightful claw of a crab will fossilize, but not millions of microscopic poison glands. This gives us a distorted perspective because the Ediacaran have no visible weapons.

But eventually the tube model did win the contest. Apparently there was a tipping point where the physical model overcame the chemical model such that bite-and-run become the dominate form. As mentioned above, trying to chew on an Ediacaran would give you nothing but a mouth full of poison. Perhaps some innovation eventually came out of nowhere that rendered chemical warfare obsolete as a primary tool. We can only speculate, but speculate we will.

Perhaps the table-turning innovation was a hardened claw or spike of some kind. It would be on the end of a limb(s) such that the body itself was not at direct risk of chemicals. A worm with teeth would not cut it; it would require a weapon that extended away from the body in some way. (Something like the Opabinia, or a crab-like animal come to mind.) The claw or spike could be made out of, or be coated with hard minerals impervious to most chemicals.

However, it still may be difficult to eat a critter that is full of chemical glands, even if it is dead. It may have required a multi-pronged strategy, such as stabbing the Ediacaran multiple times to kill it, and then waiting for it to decay enough where its poisons are not as potent. The walls of the dead poison cells would eventually decay, forming breaches, and the poisons would leach out into the surrounding fluids, and eventually into the sea. (Some may have remained such that a dead Ediacaran eater may also have to evolve some amount of poison immunity.)

A "stab and wait" strategy is fairly complex one that could have taken a while to evolve. For example, one would have to either remember where the dead body was, or be patient enough to wait out its partial decay. This was some 600 million years ago where "brains" could hardly be called brains. Stabbing a bag of poison and then waiting around instead of swimming off to find your normal food would go against normal instincts.

Unless you plan to eat it, you are not going to even bother to attack. However, the poking activity may have first evolved as a defense. Over time it may have grown sophisticated enough that there were enough fresh dead Ediacaran laying around, ready to eat. Although most Ediacarans might win the battles, a few may have lost. A branch species of "stabber" may have slowly grown to specialize in these freshly-dead Ediacarans to some degree. If it's lying around dead, you might as well eat it. The stabber's ability may have been effective enough on weak, small, or young Ediacarans to make them a staple.

Once that happens, its stabbing ability would start to improve and it would naturally hang out in the areas occupied by bigger Ediacarans rather than run away. At this point it may then evolve the instinct to wait around after a fresh kill for decay to take place, or to remember where the last stabbing took place so that it could come back later.

A Cambrian-style animal is finally able
to compete with a poisonous Ediacaran-style
animal by killing it from a distance,
triggering the Cambrian Explosion.

The stabbing critter could perhaps first have developed the stabbing ability for some other purpose, and then adapted it to the Ediacarans. But once this strategy evolved, it may have completely decimated the Ediacarans such that the tube form then filled in the vacant niches left by dead Ediacarans, triggering the Cambrian explosion full of creatures that use tube-based digestion and lots of hard, pointy limbs and claws.

Thus, according to this speculation, the Ediacarans had the most effective form of survival for dozens of millions of years using external digestion and poisons until complex strategies evolved to overpower the chemical weapons of these Ediacarans using complex behavior and the physical force of a new kind of limb.

Downsides

Other Possibilities

Combination Machine

This could be why the Ediacaran critters disappeared. Although they invented multi-cellular forms first or quicker, they were perhaps stuck with the tree model whereby each species is genetically independent. There was no significant cross-species trading. After all, mix-and-match conceptually seems more efficient than each branching having to reinvent useful idioms like eyes, fins, legs, chemical paths, etc. from scratch. Sharing allows good ideas to be swapped like baseball cards on the corner block: "I'll give you a leg if you give me an eye".

This speculative Cambrian sharing ability may have been lost over time as creatures grew too complex to easily reintegrate diverse parts, and as most of the better combinations were already taken up via past combinatorial experiments. The benefits of this cross-species genetic mechanism may have thus diminished over time such that the benefits of keeping it around disappeared and regular mutations eventually disabled it.

Current animal species generally do not exchange significant genetic material, or at least don't appear to take advantage of it when they do. (Microbe and virus infections are the most likely method if it does happen.)

This situation perhaps comparable to forked software builds (branching versions) where it is easy to barrow across forks early in the project, but it grows more difficult over time because the internal structures and idioms grow different such that selected portions may need things specific to a given fork in order to function. If you want to mix-and-match across forks, it is best to do so early in the project, just after the fork (the "Cambrian" era of the software forking).

Note that the above is kind of a variation of the "HOX gene" theory, where the HOX gene set allegedly allowed small mutations to control large changes in the animal. Thus, the gene set is like a road-show switch panel whereby you can hire a toddler to play around by flipping and turning the switches and dials in different combinations to produce different stage lighting and synthesized sound. This theory is that evolution stumbled upon an "instant body plan machine" that made evolution experiments (mutations) more fruitful and inventive.

Single Cell

Large single cells may have been the quickest way to dominate the landscape, at least in terms of size. Small single cells already existed at that time, so if you simply increase your size a little bit each generation, you don't have to do anything significantly different to be big; just incremental adjustments.

In entrepreneurship, being the first to a niche is sometimes better than being the best. Being the first allows one to grab customers and market share first such that there are profits to improve the product beyond competitors who may initially had better technology. Having a larger population gives evolution more chances to fix any problems with a given approach.

However, even in the market place there is no guarantee that being the first will keep you from being unseated, as the inventor of the computer spreadsheet can tell you. This may be what happened to the Ediacarans. Simply scaling up the size of a single cell may have been the quickest way to reach the niche of largeness, but multi-cellular forms won the longer-term race.

It's a variation of the classic Tortuous versus the Hare story. The Hare got the quick start, and that is why the Ediacaran forms were dominant for a few dozen million years. But multi-cells proved to be the better model overall; it just took longer to perfect it. Multiple cells perhaps make it easier to have differentiated tissues that specialize in different functions. This may be why the Ediacarans appear to lack limbs, digestive tracks, and eyes. Those things may have been too hard to do well using one big cell. Thus, they reached the limit of the single-cell model, or at least could not stretch it fast enough to beat off the up-coming multi's, and the rest is history.